Seepage layer determination and investigation on seepage mechanism in the left abutment of Aghchai dam (West Azarbaijan) using hydrogeological studies

Aghchay earthfill dam with clay core is located in West Azarbaijan province, 45km far from Khoy city with 111.5 m height from foundation and 179.5 million cubic meters of reservoir capacity. It was constructed on the coral limestone, marl and sandy marls of the Oligo-Miocene Qom Formation. Presence of a fault at the vicinity of the site and outcrop of Qom Formation in the abatements of the dam has been posed a risk of seepage. Two years after the construction and opening of the dam some seepage were observed at the left (one pint) and right abutments (4 points) with the overall discharge of 16 L/S. In this research besides geotechnical and geological findings, hydrogeological studies are employed to determine seepage route and mechanism. So, some samples were collected from the seeped water and dam reservoir water and were sent to laboratory for analysis of major ions. Also, the seepage discharge and its EC were monitored for one year. The results of hydrochemical analysis showed a close relationship between the water of the reservoir and the seeped water, expectedly. Hydrogeological findings and benchmarking of the seeping points revealed that the seepage has been occurred through the interface of a conglomerate and marl layer at the level of 1250 to 1270 meters above sea level. Monitoring of seepage behaviour related to the reservoir level showed that the reservoir level fluctuations changes the hydraulic head and also the seepage discharge and exposure elevation. Because that the seeped water is in under saturation condition related to halite and gypsum minerals, so increasing of seepage in the future due to solution of the marl unit is completely probable.